How do i add mist to a cube spelling something
You could use something like this. The magic ingredient here is the volume scatter node. I used a Noise Texture and a ColorRamp to give the smoke a more realistic look, and connected the volume scatter to the volume input only (no surface input). You can see on the right, the quality of the fog is determined largely by the Tile Size and number of Samples, but the Start and End range have an effect as well (making the gap smaller will improve the quality) - just remember that this gap determines the viewable distance of the volume, so keep it in a range where your camera will be. I darkened the world background in the image to make it stand out more, and I had to brighten the light to make the volume stand out (it's visibility is affected by the light it receives). I also gave the light a purple glow just to add effect. As you can see, however, some shapes help to constrain the smoke more than others. Try it out for yourself: